But Wallaby coach John Connolly confirmed on Monday that Gregan is certain to start the match alongside his long-time team-mate and fly-half, Stephen Larkham, who will make his 100th Test match appearance.
"It's a great occasion, you don't often get a guy who's (played) 130 plus (Test matches) and another guy playing his 100th Test," Connolly said.
"The contribution they've made to Australian Rugby is phenomenal and they're virtually icons of Australian sport."
Larkham will join Gregan and former Wallaby winger David Campese, as only the third Australian to win 100 international caps.
Larkham's 100th Test will also mark the end of his career on home soil, 11 years after making his international debut against Wales in Sydney in 1996.
"It's a very proud milestone for myself but to have George (Gregan) alongside me is going to make it doubly special," Larkham said.
"I think sharing the last Test in Australia with George is also going to be very special and I'm very happy that he's going to be fit for the Test."
But the Wallabies' upset victory against the All Blacks may have come at a heavy cost with number eight Wycliff Palu suffering nerve damage to his right shoulder in the first half.
Palu is considered vital to the Wallabies' World Cup hopes in September and will undergo a series of examinations this week to determine the extent of the damage.
The Wallabies will face a Springboks outfit determined to ruin their party after the Australian press labelled them a "B-team" when a number of familiar faces were left out of the touring party.
But Larkham is adamant the Wallabies won't underestimate their opponents as they look to gain a mental edge ahead of the World Cup.
"It's obviously not going to be the same side that we played over there but still, going into the World Cup I think it'll be a very good psychological boost for us if we can get the win," added Larkham.